Gravure printing is a form of intaglio printing, i.e. printing which uses a plate or cylinder into the surface of which the subject matter to be printed is etched or engraved. A liberal film of fluid printing ink is supplied to the whole printing surface and the surface is then wiped, for example by a doctor blade, in order to remove all the ink from the unindented parts of the surface leaving ink only in the indentations or cells. Paper in a continuous web or in separate sheets is then pressed into contact with the inked surface in order to receive an impression of the subject matter.
In the most widely used kind of gravure printing, which is known as the rotogravure process, the subject matter, which may be textual or pictorial, is etched into the printing surface in the form of a matrix of cells which vary in depth and/or in surface area, so that the cells corresponding to the darker parts of the subject matter have a greater capacity for ink than the cells which correspond to the lighter parts of the subject matter. An image of the subject matter is formed by a photographic process on a sheet of carbon tissue which is impregnated with gelatine containing a light sensitive reagent. There is first formed on the sheet of carbon tissue a rectilinear grid having from 59 to about 160 lines to the centimeter. The grid is formed by placing a screen consisting of small opaque squares separated by fine transparent lines in contact with the impregnated carbon tissue and exposing the screen to light so that the gelatine in the tissue immediately below the lines is rendered insoluble.
The image of the subject matter to be printed is then superimposed on the image of the screen by placing in contact with the carbon tissue a positive photographic transparency of the subject matter for the colour to be printed, and exposing the transparency to light. Again the gelatine in areas of the carbon tissue lying immediately beneath clear areas of the transparency is rendered insoluble and in other areas the solubility of the gelatine is inversely proportional to the amount of light transmitted by the transparency. The carbon tissue is then placed over the surface of a specially prepared copper roller, those parts of the gelatine which are still soluble are washed away, and the surface of the roller is etched with a suitable reagent such as ferric chloride. The result is that the surface of the cylinder is etched in a pattern composed of a very large number of cells defined by a rectilinear grid, the depth of the cells in a particular area being dependent on the solubility of the gelatine in the carbon tissue overlying that area and thus on the amount of light transmitted through the transparency in that area.
The choice of a suitable paper for gravure printing is largely empirical. Results can be obtained on a wide variety of different types of paper ranging from newsprint to the finest matt art paper. However as a general rule, the paper should be absorbent enough to take the ink without the exertion of undue pressure, and a coated paper is generally required for the best results.
the gravure printing process is especially suitable for printing runs in which a large number of copies are required because the recessed cells of a gravure cylinder are less subject to wear through abrasion than the relief type of the letterpress process.
The process is therefore used for printing magazines, mail order catalogues and other periodical publications having a large circulation. There is an increasing trend to print this type of publication on a lightweight coated paper in order to minimise postal costs. Unfortunately a very common defect which appears when subject matter is printed by gravure on lightweight coated papers is a speckled effect which is most noticeable in the middle tones. This effect is caused by poor contact between the surface of the paper and the surface of the cylinder so that the ink is not drawn out from some of the cells with the result that some of the minute dots which make up the printed image are missing.
Accordingly, the present invention arose from a need to provide a new pigment for lightweight coated magazine paper for printing by gravure only. The objective was to maximise print quality by gravure, while enabling magazine publishers to reduce the weight per page of their publications. It was and still is believed that the weight of the main body of the paper has reached its lower limit and that any further reduction would make the paper too weak. It was consequently hoped that the desired weight reduction could be achieved by reducing the coating weight. However, previous experiments has shown that a reduction in the coating weight invaribly resulted in a marked deterioration in gravure repreduction quality. The accepted minimum coating weights for satisfactory gravure reproduction were around 9 or 10 grams per square meter (g/m.sup.2).
In his research, the present inventor started from the basis that good quality printing in gravure requires a compressible paper, which would more reliably contact the printing cylinder over its entire area and so ensure that the maximum possible ink pick-up from the cylinder was achieved. The present inventor investigated whether this compressibility could be introiduced into the coating layer by using, as a pigment, a clay having a narrower particle size distribution than occurs naturally. The basis for this was that a restricted particle size distribution would result in poor packing characteristics with a consequent high void content, giving the required compressibility. This approach was a departure from conventional thinking which was that the paramount requirement for good printing by gravure methods was a smooth surface, which implied high gloss. Gloss is improved by reducing the size of the particles composing the pigment. Conventional thinking, therefore, was that acceptable coatings for gravure printing had to be made from pigments containing fine particles.
The inventor unexpectedly discovered, in accordance with the present invention, that clays with narrower particle size distribution gave surprisingly and unexpectedly good gravure reproduction at normal coating weights; and, more surprisingly, that by utilising such a clay, good gravure reproduction was maintained with coating weights as low as 7 g/m.sup.2. Another surprising result was that good reproduction was obtained in the presence of significant proportions of relatively large particles, (i.e. greater than 10 microns). This was in complete contrast to the prevailing belief that, as the particle size of the pigment increased above 5 microns equivalent spherical diameter, the gravure print quality deteriorated. This surprising ability to achieve satisfactory printing results using coarser particles is of major importance for the commercial prospects of the present invention. It means that the cost of the processing necessary to produce a good quality product can be reduced by using a coarser pigment fraction than is normally considered suitable for the production of pigments for paper coating compositions for gravure printing paper.